Activated carbon is commonly used in metallurgical processing, for example in the extraction of gold from ore. During the processing the carbon becomes contaminated through the clogging of pores in the carbon with impurities and the carbon needs to be reactivated before it can be used again. For reactivation, the carbon is heated to a temperature of approximately 700.degree. C. (i.e. the reactivation temperature) over a period of approximately 60 minutes. The heating burns the impurities which have been absorbed by the carbon.
Heating devices which are presently in use for reactivating carbon comprise gas fired kilns and electric resistance furnaces. In order to thoroughly heat the carbon, conventional gas fired kilns and electric resistance furnaces typically comprise a rotary kiln. The batch of carbon is placed in the rotary kiln and the kiln is rotated during the heating cycle in an attempt to uniformly heat the carbon. To provide uniform heating, known kiln furnaces typically include a blower which forces heated air or gas over the carbon material, which means that the furnace must have additional capacity to heat the make-up air. The advantage of the rotary kiln is reactivation of a greater percentage of the carbon in the batch is usually achieved. The disadvantage however is the mechanical breakdown of the carbon, caused by abrasion during rotation, which renders a portion of the carbon unusable due to the smaller size of the carbon. Furthermore, the addition of a blower increases the cost of the system.
In the art, vertical kiln furnaces are also known for processing carbon. Vertical kiln furnaces comprise a vertical heating chamber which is supplied with carbon through a feed tube at the top. The heating chamber includes external heating sources which heat the carbon inside the chamber. In such a furnace, the vertical flow of carbon through the heating chamber is controlled and the applied heat energy raises the temperature of the carbon to burn the impurities. The moisture and impurities which are released from the carbon tend travel up and accumulate at the top of the vertical kiln. The accumulation of moisture can cause condensation problems and the collection of impurities which may include volatile games can present a hazard, for example, due to possibility of spontaneous combustion. The accumulation at the top of such conventional vertical kilns can also clog the feed tube supplying the carbon Another disadvantage associated with conventional vertical kiln furnaces is the limitation on the width of the heating chamber and therefore the throughput of the kiln. The width of the heating chamber is limited by the thermal conductivity of the material, e.g. carbon, being heated inside the kiln.
Other disadvantages associated with conventional kilns include the need to maintain the kiln at a high temperature even during idle periods. Because there is a lag time to raise the temperature in the furnace, the furnace is typically kept at an elevated temperature during idle periods in order to provide an acceptable response time when loaded with material for processing. It will be appreciated that continuously maintaining the furnace at an elevated temperature can consume considerable energy.
The present invention overcomes these disadvantages. The present invention utilizes a microwave energy source to directly heat the granular material, e.g. carbon, in a two-chamber apparatus. The first chamber provides a drying and preheating zone in which the material is heated to a first temperature in order to decrease the moisture content and for some materials, e.g. ceramics, increase the microwave coupling of the material. The benefits of microwave heating, i.e. rapid heating with lower energy consumption, are realized in the second chamber where further microwave heating is applied to the material to reach the reactivation or regeneration temperature. The present invention also provides an apparatus for continuously heating the granular material.